Glow tube, in particular for a sheathed-element glow plug

ABSTRACT

A glow tube, in particular for a sheathed-element glow plug. The glow tube, which has a dome-shaped, closed end section ( 27 ) and a cylindrical, open end section ( 28 ), is composed of at least two axially serially disposed glow tube parts. The one glow tube part is formed at the closed end section ( 27 ) by a seamless glow tube cap ( 31 ), and the other glow tube part is formed at the open end section ( 28 ) by a substantially cylindrical glow tube extension ( 32 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a glow tube, in particular for a sheathed-element glow plug.

2. Description of the Related Art

Glow tubes are used in sheathed-element glow plugs and, there, form the outer shell of a glow element that extends into the combustion chamber of an internal combustion engine. A sheathed-element glow plug of this kind is known, for example, from the published German utility model application document DE 94 12 268 U1. The glow element is sealingly fixed, together with the glow tube, by a portion of its length, in a longitudinal bore of a tubular metal housing. At its end proximal to the combustion chamber, the thin-walled glow tube is sealed by a dome-shaped end section and, at the opposite end, by an open, cylindrical end section. In the interior of the glow tube, an axially extending resistor element used as a heating device is embedded in a ceramic insulation material. The resistor element is provided distally to the combustion chamber with a connector for an electric circuit. In response to closing of the electric circuit, the heating device heats up, and the glow tube of the sheathed-element glow plug heats to an operating temperature of at least 1100° C. during each glow process. Therefore, the material of the glow tube is subject to a very intense thermal stress. Ni-based alloys are used as material for the glow tube, for example.

Longitudinal-seam welded tubes are typically used as glow tubes. The starting product for these tubes is a sheet-metal strip that is bent into a tube and is welded at the joint without any filler. To eliminate welding stress, the tubes are subsequently annealed. The tubes are then drawn over a mandrel, the degree of deformation being 10 to 50%. A recrystallization annealing subsequently follows again in order to adjust the mechanical properties and the structure.

Modern diesel engines are required to start without any appreciable preheating. For that reason, the sheathed-element glow plugs must heat up very rapidly. In order for the Diesel engines to achieve efficient cold-running properties, sheathed-element glow plugs featuring higher glow temperatures are needed. Higher glow temperatures produce, in turn, high temperature gradients, so that, under these operating conditions and following an extended period of stress, the glow tube exhibits cracks along the weld seam. This pattern of failure is caused by thermomechanical stresses. Due to the different structure of the weld seam, the sheet-metal material of the glow tube and the heat-affected welding zone, the strains induced by the thermomechanical loads are concentrated at the mechanically weakest region, namely the heat-affected welding zone.

To avoid such thermomechanical stresses in the weld seam zone, the published German patent application document 102008010243.1 already discusses using seamless glow tubes. Using customary deepdrawing techniques, it is impossible or difficult to manufacture glow tubes of this kind having a diameter of 4 to 7 mm, for example, and a length of 30 to 45 mm, for example.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to devise a glow tube that will withstand higher thermal stresses and be able to be manufactured at a cost that is reasonable from a production engineering standpoint.

The glow tube design of the present invention, which provides for at least two mutually joined glow tube parts having a dome-shaped glow tube cap and a substantially cylindrical glow tube extension, makes it possible for current production processes to be used to produce the end section of the glow tube that points into the combustion chamber and that is thermally the most heavily stressed end section of the glow tube. The cylindrical glow tube extension that is contiguous thereto may then be designed in the manner of customary longitudinal seam-welded glow tubes or be seamlessly fabricated from a less temperature-resistant, but more readily cold-formable material. This makes it possible to use an established manufacturing method to implement the seamless glow tube cap which has only a small depth.

In the case of a sheathed-element glow plug, the joint for the glow tube cap and the glow tube extension is expediently at a glow tube location that is subject to less thermal stress than the major portion of the glow tube cap. The joint is formed by an annular end face formed on the glow tube cap and another annular end face formed on the glow tube extension. The glow tube cap and the glow tube extension are joined at the joint by a circumferentially extending weld seam in an integral bond.

The glow tube cap has a closed dome and a substantially cylindrical section. The axial length of the cylindrical section is, at a maximum, eight times the diameter of the cylindrical section following the hammering process used for the glow tube, i.e., in the finish-assembled state. Thus, a favorable diameter-to-length ratio of ½ to ⅓ is provided for the deepdrawing of the glow tube cap, these specifications referring to a glow tube cap prior to the hammering process.

The multipart design of the glow tube makes it possible for the thermally less heavily stressed glow tube sections to be manufactured out of a less expensive material.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the present invention is illustrated in the drawing and explained in greater detail in the following description.

FIG. 1 shows a basic design of a sheathed-element glow plug according to the related art.

FIG. 2 a shows two glow tube parts of the glow tube according to the present invention.

FIG. 2 b shows the glow tube parts according to FIG. 2 a in the assembled state.

DETAILED DESCRIPTION OF THE INVENTION

The sheathed-element glow plug illustrated in FIG. 1 has a tubular housing 11 having a longitudinal bore 12 within which a glow element 13 is sealingly fixed by a portion of the length thereof. Glow element 13 has a thin-walled glow tube 14 having an inside diameter of 3 to 7.5 mm, for example, and a wall thickness of 0.5 to 1.0 mm.

Extending axially in interior 16 of glow tube 14 is a resistor element 17 used as a heating device that is embedded in a ceramic insulation material 18. Resistor element 17 is provided distally to the combustion chamber with a connector 19 for connecting an electric circuit. The end of resistor element 17 that is proximal to the combustion chamber is electroconductively connected to the bottom of glow tube 14.

Resistor element 17 is composed of two series connected resistance coils, for example, the resistance coil proximal to the combustion chamber forming a heater coil, and the resistance coil distal to the combustion chamber forming a control coil 21. Due to its high positive temperature coefficient, control coil 21 has the task of limiting the current in the electric circuit in response to a rise in temperature.

In accordance with FIG. 1, glow tube 14 has a closed, dome-shaped end section 27 and an open, cylindrical end section 28. In accordance with the present invention, glow tube 14 has a two-part design. In accordance with FIG. 2 a and FIG. 2 b, glow tube 14 is formed at closed end section 27 by a seamless glow tube cap 31 and, at open end section 28, by a cylindrical glow tube extension 32.

As is the case for the previous glow tubes, cylindrical glow tube extension 32 is fabricated from a sheet-metal strip that is bent into a tube and is welded at an axial joint 33. However, cylindrical glow tube extension 32 may also be fabricated from a more readily cold-formable, but less temperature-stable material. Disposed oppositely to open end section 28 is an annular end face 34 on glow tube extension 32.

Glow tube cap 31 has a seamless design; i.e., it does not have any weld seam in the axial direction. At one closed end section, glow tube cap 31 is closed by a dome 35. Disposed contiguously to dome 35 is a short cylindrical section 36, which, as a function of the manufacturing technique for glow tube cap 31, has a length l of 10 to 30 mm and a diameter d of 3 to 7 mm. Dome 35 forms the tip of glow element 13. Cylindrical section 36 terminates with an annular end face 37 at glow tube cap 31.

Seamless glow tube cap 31 is manufactured, for example, by the deepdrawing of a sheet metal. The relatively short depth of glow tube cap 31 renders possible a diameter-to-length ratio that is favorable for the deepdrawing process which, in the present exemplary embodiment, is ½ to ⅓, these specifications referring to a glow tube cap 31 prior to the hammering process. However, it is equally possible to produce glow tube cap 31 using a machining process.

In accordance with FIG. 2 b, glow tube cap 31 and glow tube extension 32 are joined at end faces 34, 37 at a joint and welded at the joint by a circumferentially extending weld seam 39.

The use of a longitudinally welded tube for cylindrical glow tube extension 32 is possible because the sheathed-element glow plug is subject to less thermal stress due to the temperature drop toward the end facing away from the combustion chamber. Such cylindrical glow tube extensions 32 may be produced in any given length without any loss of quality, which, in turn, allows glow tubes 14 of any given length to be manufactured that are composed of at least two parts. Besides the method for manufacturing longitudinally welded tubes, a more cost-effective material may be used for glow tube extension 32 than for the thermally highly stressed glow tube cap 31. Glow tube cap 31 is to be manufactured from a material that withstands higher thermal stresses. The less thermally stressed section of glow tube 14 that is formed by glow tube extension 32 may be manufactured from a material that only needs to withstand a slight thermal stress. The less expensive glow tube extension 32 may also be designed as a variable component having different lengths, for example, the same glow tube cap 31 being usable for all possible variants.

Besides the described two-part design of glow tube 14, it is also conceivable for glow tube 14 to be manufactured out of more than two glow tube parts. The axial position of a seamless glow tube cap 31 may be varied as needed. Depending on the temperature profile along glow tube 14, seamless glow tube cap 31 may be variably positioned. However, glow tube cap 32 described in the exemplary embodiment is advantageous at the side of glow tube 14 facing the combustion chamber because, in this case, no drastic changes in the design of the sheathed-element glow plug are necessary. Moreover, it is also conceivable for glow tube 14 to be used for other high temperature-stressed protective sleeves for active or passive devices (for example, temperature-measuring probes, sensors, etc.) 

1-7. (canceled)
 8. A glow tube for a sheathed-element glow plug, having a dome-shaped closed end section and a cylindrical open end section, the glow tube comprising: at least two glow tube parts, wherein one glow tube part is formed at the closed end section by a seamless glow tube cap, and the other glow tube part is formed at the open end section by a substantially cylindrical glow tube extension.
 9. The glow tube as recited in claim 8, wherein a joint is formed between the glow tube cap and the glow tube extension, and the joint is situated at a location of the glow tube that is subject to less thermal stress than the major portion of the glow tube cap.
 10. The glow tube as recited in claim 9, wherein the joint is formed by an annular end face formed on the glow tube cap and another annular end face formed on the glow tube extension.
 11. The glow tube as recited in claim 9, wherein the glow tube cap and the glow tube extension are joined at the joint in an integral bond.
 12. The glow tube as recited in claim 10, wherein the glow tube cap and the glow tube extension are joined at the joint in an integral bond.
 13. The glow tube as recited in claim 8, wherein the glow tube cap has a closed dome and a substantially cylindrical section, and the axial length of the cylindrical section is, at a maximum, eight times the diameter of the cylindrical section following the hammering process used for the glow tube.
 14. The glow tube as recited in claim 9, wherein the glow tube cap has a closed dome and a substantially cylindrical section, and the axial length of the cylindrical section is, at a maximum, eight times the diameter of the cylindrical section following the hammering process used for the glow tube.
 15. The glow tube as recited in claim 10, wherein the glow tube cap has a closed dome and a substantially cylindrical section, and the axial length of the cylindrical section is, at a maximum, eight times the diameter of the cylindrical section following the hammering process used for the glow tube.
 16. The glow tube as recited in claim 8, wherein the glow tube cap and the glow tube extension are made of different materials.
 17. The glow tube as recited in claim 9, wherein the glow tube cap and the glow tube extension are made of different materials.
 18. The glow tube as recited in claim 10, wherein the glow tube cap and the glow tube extension are made of different materials.
 19. The glow tube as recited in claim 13, wherein the glow tube cap and the glow tube extension are made of different materials.
 20. The glow tube as recited in claim 8, wherein the glow tube cap is produced by deepdrawing, MIM (metal injection molding), powder extrusion, metal injection casting or by machining.
 21. The glow tube as recited in claim 9, wherein the glow tube cap is produced by deepdrawing, MIM (metal injection molding), powder extrusion, metal injection casting or by machining.
 22. The glow tube as recited in claim 10, wherein the glow tube cap is produced by deepdrawing, MIM (metal injection molding), powder extrusion, metal injection casting or by machining.
 23. The glow tube as recited in claim 13, wherein the glow tube cap is produced by deepdrawing, MIM (metal injection molding), powder extrusion, metal injection casting or by machining. 